Conventionally, a heating coil for partially induction-heating a plurality of places at sides of a work has been proposed (see Patent Literature 1). For example, the heating coil 40 is well-known as such a kind of coil, which has a pair of heating conductors 41, one of which is arranged at a side of a work 10 and the other one is arranged at an opposite side of the work 10; and an end of heating conductors 41 is respectively connected with a connecting conductor 42, as shown in FIG. 7.
In the heating coil 40, the work 10 is arranged between the pair of heating conductors 41; the heating conductors 41 faces a part of sides of the work 10; a power source is connected with the other end of the pair of heating conductors 41; and power is fed through heating conductors 41 and the connecting conductor 42 to perform partial induction heating on respective sides of the work 10.